T4 (Thyroxine), Free, Serum
Use
Free thyroxine (FT4) is the metabolically active fraction of thyroxine, comprising a small part of the total thyroxine. It is available to the tissues and is crucial for evaluating thyroid function disorders. Elevated FT4 levels are indicative of hyperthyroidism, whereas decreased levels suggest hypothyroidism. The test is often used in conjunction with thyroid-stimulating hormone measurements to assess thyroid disorders.
Special Instructions
Patients should not take multivitamins or dietary supplements containing biotin (vitamin B7) for 12 hours before specimen collection. This is commonly found in hair, skin, and nail supplements, as well as multivitamins. If not ordering electronically, complete and send a Renal Diagnostics Test Request form with the specimen.
Limitations
The test cannot be used in patients receiving treatment with lipid-lowering agents that contain dextrothyroxine. It should be discontinued for 4 to 6 weeks before thyroid function is checked to ensure accurate results. Interference can occur due to high titers of antibodies, heterophile antibodies, or human antimouse antibodies, potentially leading to false readings. The presence of familial dysalbuminemic hyperthyroxinemia can also cause anomalies. Among 26 commonly used pharmaceuticals, only furosemide may cause elevated FT4 findings at therapeutic levels.
FDA ApprovedNew York Approved
Methodology
Immunoassay (ECLIA)
Biomarkers
LOINC Codes
- 83122-2
- 83122-2
Result Turnaround Time
1-2 days
Related Documents
For more information, please review the documents below
Specimen
Serum
Volume
1 mL
Minimum Volume
0.5 mL
Container
Plastic vial
Collection Instructions
Centrifuge and aliquot serum into a plastic vial.
Patient Preparation
For 12 hours before specimen collection, do not take multivitamins or dietary supplements containing biotin (vitamin B7).
Causes for Rejection
Gross hemolysis, gross lipemia, or gross icterus will not cause rejection.
Stability Requirements
| Temperature | Period |
|---|---|
| Room Temperature | 72 hours |
| Refrigerated | 14 days |
| Frozen | 30 days |